For forced indiction applications, I've read it's a good idea to start with a low compression engine to safely use more boost. What compression are you guys running with what boost?

 

9.5/1 w/Paxton Novi 2000 making 15-18psi of boost...

 

Be careful how low you go. Too little compression means you need to make lots more boost, which needs a better belt/pulley/bracket setup. It also makes it feel softer at lower rpm and the bigger belt load means more power is consumed to make power.

You can make tons of power at 8.5:1cr, but it's much less work to make the same power at 9.5:1cr. Conversely boost can add power to a 10.5:1cr motor, but it can add much more to a 9.5:1cr motor.

I think 9-9.5:1 is probably about right for a high-hp motor.

 

10.9:1 with a Maggie at 5.2 psi boost.

 

10.8 9lbs boost 575/525 383 A4 A&A kit, great throttle response.

 

what if my car is at 4500ft elevation? I was told to go 1 point higher then sea level. I am thinking of building my motor for my STS setup but feel I will go back to a supercharger down the road....because I like a loud exhaust. 15-18# is my goal.

 

6.8 LBS B@@ST ProCharger 3.85" Pulley on 10.9:1 64cc TFS Heads!

Thanks,Matt

 

 

Yes the norm is to run between 9:0 to 9:5 to 1 compression for must supercharged applications. However, you can run higher compression if tuned right and if the motor can handle it, and just like my stock LS6 motor that has 10:5 to 1 compression (and 11,700 trouble free miles) my forged LS6 blower motor being built, will also be 10:5 to 1, as this will make BIG POWER with less boost, and CRAZY power with the 12 lbs of boost I expect to run (should make 700rwhp) and the mid range power will be outstanding also, as well as the responsivnes of the the car off the boost, stop light to stop light!

 

9.7:1CR, 383ci, 8 psi, 605rwhp/540rwtq, built by me and tuned by ECS

 

Low compression will give you a soft bottom end just driving around. I think Dynamic compression should be more of a consideration. I run 12.0:1 compression on my NA motor. My cam is so large with a wide lobe seperation that I only have 8.6:1 DCR. The car runs fine on 93 octane in 101* humid heat. On my next SC car I am going to run the same cam and probably around 10.0-10.5:1. I am willing to bet my DCR will actually be much lower than my LS6 motor with a 224/228 cam that I ran at 10psi on my stock motor. I may actually find out what the DCR was on the LS6 motor and adjust my compression ratio to match it. Now that I am forged I will run 12-14lbs of boost.

 

You know I hear this alot and am yet to see it play out on the Vette's in practice.

I have now built a 347, a 382, and a 427 with TT's on it. All of these motors are right about 8.8:1 compression and NONE of them are soft on the bottom end.

Also have a 416 in the works. I think it is too general of a statement to say that with a low compression motor the power will be soft down low, IMO. My real world experience tells me otherwise. These are all well over 700 HP setups.

 

That is awsome that you have your combos dialed in like that. Maybe I sould have said, softer than if you had some compression. I think it also depends on the cam and dynamic compression like I stated above.

Compression will give you better gas milage, throttle response, and power. What happend in the 70's when we went to low compression motors? They were dogs. Horse power was dead. Now motors come from the factory with 11.0:1 and we are eons ahead of the 60's era cars in power. 90% of the time a S/C motor is not in boost. Only under WOT is boost introduced. So you are essentialy running a NA motor 90% of the time. A point of compression is worth about 14hp and a lb of boost is worth about 22hp. So why not run an extra point of compression to make the NA side of things happy and one or two less lbs of boost up top. Or put some meth on the thing and let it eat. If I was going to throw 25+lbs of boost at something then I would def opt for the high eights on compression. But the truth of it is that MOST people that boost a LS engine only go to 14-15lbs max. I would actually say that the majority never see over 12lbs. Just my opinion.

Here is something I just checked on. With my stock LS6 engine with a 224/228 .581/.588 114+4 the DCR comes to 8.41:1. I ran 10lbs of boost on this engine and made 635/550rw at 6,200rpm with a restrictor plate. Now with my new cam 244/248 .612/.615 114+4 it needs 11.6:1 SCR to get close to the same 8.42:1 DCR. So this cam with and 9.5:1 compression it would most def not have the same power down low or any where else for that matter. If I am running the same DCR, which is all the motor really sees any way, then what is the harm in running 11.6:1 SCR?

 

9:4:1 @ 15 psi

 

I 100% and that is why I will be running around 10:5 to 1 compression on my forged LS6 motor that will see about 12 lbs of boost and should make BIG POWER on the boost, and be very responsive off the boost.

Also, here is a quote I saved from SLOWHAWK a well known and respected tuner here regarding compression as it relates to bigger cams and making good midrange power!


"Compression dictates Cam selection too.
Compression is key to making midrange power and with the bigger Cams the DCR lowers which makes it safe to run pump gas.
I would expect around 700hp on my dyno with this setup at 12lbs of boost."

 

I just edited my above post with some interesting information along the lines of what you just said. Give it a quick read.

 

9:6:1 14lbs right now

 

When I replaced my stock LS1 heads with some stock LQ9 heads with the 72cc chambers and changed nothing else and dropped my compression down to 9.6:1, I lost low end response, Max. power, and fuel milage. This is at 10 psi of boost on a maggie. So it does hurt power and fuel economy. The only advantage to run low compression is to run higher boost levels. If you are planning on running 15 lbs of boost, then I would think somewhere between 9:1 to 9.5:1 would be good. But the DCR from the cam timing will also play a part.